Portland Cement

MILITARY COLLEGE OF ENGINEERING
RISALPUR
CE 308 – LECTURE 2
PORTLAND CEMENT AND
HYDRATION

• The use of cementing materials is very old.
• The ancient Egyptian used calcined impure gypsum.
• The Greeks and the Romans used calcined limestone and later
learned to add lime and water, sand and crushed stone or brick
and broken tiles.
• This was the first concrete in history.
Historical Note

• Lime mortar does not harden under water, and for
construction under water the Romans ground together lime
and a volcanic ash or finely ground burnt clay tiles.
• The active silica and alumina in the ash and tiles combined
with the lime to produce, Pozzolanic cement from name of
village of Pozzuloi, near Vesuvius, where the volcanic ash
was first found.
• Some of the Roman structures in which masonry was bonded
by mortar, such as the Coliseum in Rome have survived to
this day, with the cementious material still hard and firm.

• Portland cement patented by Joseph Aspdin, in 1824.
• Prepared cement is the name given to a cement obtained
by intimately mixing together calcareous and
argillaceous, or other silica-, alumina- and iron oxide-
bearing materials, burning them at a clinkering
temperature, and grinding the resulting clinker
• The name Portland cement---resemblance of color to
Portland stone –Limestone quarried in Dorset.
• Portland cement is a fine powder that when mixed with
water becomes the glue that holds aggregates together in
concrete.
Portland cement

• Portland cements are hydraulic cements composed primarily
of hydraulic calcium silicates . Hydraulic cements set and
harden by reacting chemically with water.
• During this reaction, called hydration, cement combines with
water to form a stone like mass, called paste.
• When the paste (cement and water) is added to aggregates
(sand and gravel, crushed stone, or other granular material) it
acts as an adhesive and binds the aggregates together to form
concrete, the world’s most versatile and most widely used
construction material.
• Fresh cement is like a fine powder which is commonly
available in sealed bags. On opening the bag, cement starts
absorbing moisture from the air and with the passage of time,
small and large lumps are formed, which results in the
decline of cement strength. So the cement bag once opened
should be used as soon as possible.
Portland cement

Lime (CaO) 60 to 67%
Silica (SiO2) 17 to 25%
Alumina (Al2O3) 3 to 8%
Iron oxide (Fe2O3) 0.5 to 6%
Magnesia (MgO) 0.1 to 4%
Sulphur trioxide (SO3) 1 to 3%
Alkalis (Na2O+K2O) 0.2 to 1.3%
The chief chemical constituents of
Portland cement

Compound Oxide (Abvn) %age
(a) Tricalcium silicate 3 CaO.SiO2 (C3S) 40%
(b) Dicalcium silicate 2CaO.SiO2 (C2S) 30%
(c) Tricalcium aluminate 3CaO.Al2O3 (C3A) 11%
(d) Tetracalcium aluminate 4CaO.Al2O3.Fe2O3 (C3AF) 11%
Component Oxides

HYDRATION OF CEMENT
The hydration of cement compounds is exothermic.
The reaction proceeds slowly for 2 – 5 hours (called induction
or dormant period).
Quantity of heat evolved upon complete hydration at a given
temperature is known as heat of hydration.
For the usual range of Portland cements, about one-half of the
total heat is liberated between 1 and 3 days, about three-quarters
in 7 days, and nearly 90 percent in 6 months.
The temperature at which hydration occurs greatly affects the
rate of heat development, which for practical purposes is more
important than the total heat of hydration.

HYDRATION PROCESS
When portland cement is dispersed in water, the calcium
sulfate and the high-temperature compounds of calcium begin
to go into solution, and the liquid phase gets rapidly saturated
with various ionic species.
As a result of interaction between calcium, sulfate, aluminate,
and hydroxyl ions within a few minutes of cement hydration,
the needle-shaped crystals of calcium trisulfoaluminate
hydrate, called ettringite, first make their appearance.

HYDRATION PROCESS
A few hours later, large prismatic crystals of calcium
hydroxide and very small fibrous crystals of calcium
silicate hydrates begin to fill the empty space formerly
occupied by water and the dissolving cement particles.
After some days, depending on the alumina-to-sulfate
ratio of the portland cement, ettringite may become
unstable and will decompose to form
monosulfoaluminate hydrate, which has a hexagonal-
plate morphology.

HYDRATION REACTIONS
Hydration of Calcium Aluminates
Hydration of Calcium Silicates

DEVELOPMENT OF STRENGTH
 When water is added to cement, C3A is the first to react and
cause initial set. It generates great amount of heat.
 C3S hydrates early and develops strength in the first 28 days. It
also generates heat.
 C2S is the next to hydrate. It hydrates slowly and is responsible
for increase in ultimate strength.
 C4AF is comparatively inactive compound.
At the age of about one year, the two compounds (C3S AND
C2S) contributes approximately equally to the strength of
hydrated cement.

Setting
• This is the term used to describe the stiffening
of the cement paste.
• Broadly speaking, setting refers to a change from a fluid
to a rigid stage.
• Although, during setting, the paste acquires some
strength, for practical purposes it is important to
distinguish setting from hardening, which refers
to the gain of strength of a set cement paste.

False Set / Flash Set
False set is the name given to the abnormal premature
stiffening of cement within a few minutes of mixing with
water.
It differs from flash set in that no appreciable heat is
evolved, and remixing the cement paste without addition of
water restores plasticity of the paste until it sets in the
normal manner and without a loss of strength.

CEMENT TESTS
Because the quality of cement is vital for the production of
good concrete, the manufacture of cement requires stringent
control.
A number of tests are performed in the cement plant laboratory
to ensure that the cement is of the desired quality and that it
conforms to the requirements of the relevant national standards.
These tests include:
• Fineness tests
• Consistency of standard paste
• Setting times
• Soundness tests
• Strength tests

CEMENT TESTS
Fineness of Cement.
Since hydration starts at the surface of the cement particles.
Fineness of cement is the total surface area of cement grains
available for hydration (expressed as specific surface m2/kg)
The rate of hydration depends on the fineness of cement
particles, and for a development of strength a high fineness is
necessary (e.g. minimum 325 m2/kg fineness is required for rapid
hardening cement).
Greater the fineness more is the surface available for hydration,
resulting greater early strength and more rapid release of heat.
An increase in fineness increases the amount of gypsum
required for retardation because, in a finer cement, more C3A is
available for early hydration.

CEMENT TESTS
Consistency of standard paste.
Consistency indicates the degree of density or stiffness of
cement.
For tests like initial setting time, final setting time, and for Le
Chatelier soundness test, neat cement paste is required.
A certain minimum quantity of water is required to be mixed
with cement so as to complete chemical reaction between water
and cement, less water than this quantity would not complete
chemical reaction thus resulting in reduction of strength and more
water would increase water cement ratio and so would reduce its
strength.
So correct proportion of water to cement is required to be
known to achieve proper strength while using cement in structure.
This can be found out knowing standard consistency of cement
paste.

CEMENT TESTS
Setting times.
This is the term used to describe the stiffening of the cement
paste.
Broadly speaking, setting refers to a change from a fluid to a
rigid stage.
Setting is mainly caused by a selective hydration of C3A and
C3S and is accompanied by temperature rises in the cement paste;
initial set corresponds to a rapid rise in temperature and final set
corresponds to the peak temperature
Although, during setting, the paste acquires some strength, for
practical purposes it is important to distinguish setting from
hardening, which refers to the gain of strength of a set cement
paste.

CEMENT TESTS
Soundness.
It is essential that cement paste, once it has set, does not
undergo a large change in volume.
In particular, there must be no appreciable expansion which
could result in a disruption of the hardened cement paste.
This is ensured by limiting the quantities of free lime and
magnesia which are causing change in volume of cement (known
as unsound).
Free lime is present in clinker and is intercrystallized with other
compounds; consequently it hydrates very slowly occupying a
large volume than the original free calcium oxide.
Soundness of cement may be tested by Le- Chatelier method.

CEMENT TESTS
Strength.
Strength tests are not made on neat cement paste because of
difficulties in obtaining good specimens and in testing with a
consequent large variability of test results.
Cement-sand mortar and in some cases concrete of prescribed
proportions made with specified materials under strictly
controlled conditions, are used for the purpose of determining
strength of cement.
There are several forms of strength tests,: direct tension,
compression and flexure. In recent years the tension test has been
gradually superseded by the compression test.

TYPES OF PORTLAND CEMENT
Ordinary Portland Cement (Type I).
This is by far the most common cement used in general concrete
construction when there is no exposure to sulphate in the soil or in
ground water.
Over the years there have been changes in the characteristics of
the ordinary Portland cement: modern cements have a higher C3S
content and a greater fineness than 40 years ago.
Rapid Hardening Portland Cement (Type III)
It is similar to Type I, but the strength of this cement develops
rapidly because of a higher C3S content (up to 70 %) and a higher
fineness (325 m2/kg).
It is used when formwork is to be removed early for re-use or
where sufficient strength for further construction is required
quickly.

Rapid Hardening Portland Cement (Type III).
It should not be used in mass construction or in large structural
section because of its higher rate of heat of hydration.
However, for constructions at low temperatures, the use of this
cement may provide a satisfactory safeguard against early frost
damage.
Special Rapid Hardening Portland Cement
It is specially manufactured cement which is highly rapid
hardening. For instance, ultra-high early strength Portland cement
is permitted to be used in UK.
The high early strength is achieved by higher fineness (700 to
900 m2/kg) and a higher gypsum but this does not affect the long
term soundness
Typical uses are urgent repairs and early prestressing.

Traditional Classification European Classification
British American [BS 8500–1: 2006]
Ordinary Portland
[BS 12]
Type I
[ASTM C 150]
Type (CEM) I
Rapid-hardening
Portland
[BS 12]
Type III
[ASTM C 150]
Type IIA
Low-heat Portland
[BS 1370]
Type IV
[ASTM C 150]
Modified cement
Type II
[ASTM C 150]
Type IIB-S
Several types of Portland cement are available commercially, and
additional special cements can be produced for special uses. Table
in the slide lists the main types of Portland cement as classified by
BS, ASTM and new BS EN Standards.

Traditional Classification European Classification
British American [BS 8500–1: 2006]
Sulfate resisting
Portland (SRPC)
[BS 4027]
Type V
[ASTM C 150]
Portland blast-
furnace (Slag
cement)
[BS 146]
Type IS, Type S
Type I (SM)
[ASTM C 595]
Type IIB-V
High slag blast-
furnace
[BS 4246]
--- Type IIB+SR
White Portland
[BS 12]
--- Type IIIA
Portland-pozzolan
[BS 6588; BS 3892]
Type IP, Type P
Type I (PM)
[ASTM C 595]
Type IIIA+SR

Low Heat Portland Cement (Type IV).
Developed in the US for use in large gravity dams, this cement
has a low heat of hydration.
Because of low contents of C3S and C3A there is a slow
development of strength than the ordinary Portland cement, but
the ultimate strength is unaffected.
The fineness must not be less than 320 m2/kg to ensure a
sufficient rate of gain of strength.
Modified cement (Type II)
In some applications a very low early strength may be
disadvantage and for this reason a modifies cement was developed
in the US.

Modified cement (Type II)
This cement has a higher rate of heat development than the type
IV cement, and a rate of gain strength similar to that of type I
cement.
It is recommended for structures where a moderately low heat
generation is desirable or where moderate sulphate attack may
occur.
Sulphate Resisting cement (Type V)
This cement has a low C3A content so as to avoid sulphate attack
from outside the concrete; otherwise the formation of calcium
sulphoaluminate and gypsum would cause disruption of the
concrete due to an increased volume of the resulting compounds.
Sulphate attack is greatly accelerated if accompanied by
alternate wetting and drying e.g. in marine structures subjected to
tide or splash.

Sulphate Resisting cement (Type V)
To achieve sulphate resistant, the C3A content is limited to 3.5 %
In US there exists cement with moderate level of sulphate
resistance. These are produced by blending Portland cement with
pozzolan, they have the C3A content limited to 8 %.
The heat developed by this cement is not much higher than that
of low-heat cement, which is an advantage, but the cost of the
former is on higher side, due to the special composition of raw
materials.
Thus, in practice the sulphate resisting cement should be
specified only when necessary; its not a cement for general use.

Portland Blast-Furnace cement
This type of cement is made by intergrinding or blending
Portland cement clinker with ground granulated blast furnace slag
(ggbs), which is a waste product in the manufacture of iron; thus,
there is a lower energy consumption in the manufacture of cement.
Slag contains lime, silica and alumina but not in the same
proportions as in the Portland cement and its composition can vary
a great deal.
Sometimes this cement is referred as slag cement
Typical uses are in mass concrete because of a lower heat of
hydration and in sea-water construction because of a better
sulphate resistance (due to a lower C3A content) than with the
ordinary Portland cement.
It is commonly used in countries where slag is widely available,
and can be considered as a cement for general use.

White and coloured Portland cements
For architectural purposes, white concrete or particularly in
tropical countries a colour paint finish is sometimes required. For
these purposes white cement is used.
It is also used for its low content of soluble alkalis so that
staining is avoided.
White cement is made from china clay, which contains little iron
oxide and manganese oxide, together with chalk or lime stone free
from impurities.
In addition special precautions are required during the grinding
of clinker so as to avoid contamination. For these reasons the cost
of white cement is high (twice that of Portland cement).

Portland-Pozzolan cements
This cement is made by intergrinding or blending pozzolans with
portland cement.
ASTM C 618-93 describes a Pozzolan as a siliceous or siliceous
and aluminous material which in itself possesses little or no
cementitious value but will, in finally divided form and in the
presence of moisture, chemically react with lime at ordinary
temperatures to form compounds possessing cementitious
properties.
It gains strength slowly and therefore require curing over a
comparatively longer period but the long term strength is high.
It is recommended for general construction. It is cheaper than the
normal Portland cement.

Portland Cement

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Portland Cement